1. Field of the Invention
The present invention relates to a terrain awareness system (TAS) and more particularly to a system for alerting a pilot of an aircraft of a dangerous flight condition which monitors the position as well as the trajectory of an aircraft based upon a satellite-based navigation system, such as a global positioning system (GPS), to provide a LOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrain advisory and warning indications based upon stored terrain data which provides relatively longer warning times than known ground proximity warning systems while minimizing nuisance warnings.
2. Description of the Prior Art
Various systems are known in the art that provide warnings and advisory indications of hazardous flight conditions. Among such systems are systems generally known as ground proximity warning systems (GPWS) which monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that inadvertent contact with terrain is imminent. Among the flight conditions normally monitored by such systems are radio altitude and rate, barometric altitude and rate, air speed, flap and gear positions. These parameters are monitored and an advisory signal and/or warning signal is generated when the relationship between the parameters is such that terrain impact is likely to occur. Typical examples of such systems are disclosed in U.S. Pat. Nos. 3,715,718; 3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221; 3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and 4,319,218, all assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference.
While the above-referenced systems do provide advisory and warning signals in the event of proximity to terrain, the warnings generated by such systems are based solely upon flight conditions of the aircraft and do not utilize any navigational information. Consequently, the sensitivity of such systems must be adjusted to provide adequate warnings when a hazardous flight condition exists without generating false or spurious warnings. However, such an adjustment can result in a compromise that may still result in nuisance warnings over terrain unique to particular geographic areas and shorter than desired warning times in yet other geographic areas.
Several attempts have been made to improve upon such ground proximity warning systems utilizing ground-based navigational information. For example, U.S. Pat. Nos. 4,567,483; 4,646,244; 4,675,823; and 4,914,436 all disclose ground proximity warning systems which monitor the position of the aircraft relative to stored terrain data in order to provide modified ground proximity warnings. However, the utility of such systems is limited. For example, the systems disclosed in U.S. Pat. Nos. 4,567,483 and 4,914,436 disclose ground proximity warning systems which utilize navigational data to modify predetermined warning envelopes surrounding certain particular airports.
U.S. Pat. Nos. 4,646,244 and 4,675,823 disclose terrain advisory systems which utilize various ground-based navigational inputs and stored terrain data to provide various ground proximity warning systems based on the position of the aircraft. In order to conserve memory, the terrain data is modeled in various geometric shapes as a function of the elevation of the terrain within the area defined by the geometric shape. The warning signals are generated as a function of the position of the aircraft relative to the model. While such systems do provide adequate terrain warnings, such systems may cause nuisance warnings under certain conditions since the predicted trajectory of the aircraft is not taken into consideration.
Another problem with such systems is that they are based upon ground-based navigational systems, such as VOR/DME and LORAN. Such ground-based navigational systems are being replaced with relatively more accurate satellite systems, such as the global positioning system (GPS). Moreover, such ground-based systems are only able to provide two-dimensional position data. Thus, with such systems, the altitude of the aircraft must be supplied by an auxiliary device, such as an altimeter, which increases the complexity of the system as well as the cost.